Two pilot studies suggested that genetic factors (in interaction with environmental factors) play a contributory role in the etiology of criminality. One study examined concordance for criminality in all of the 3,586 twin pairs born between 1880 and 1910 in a well-defined area of Denmark. The pairwise concordance rates for criminality for the monozygotic twins was 36%, for the dizygotic twins 13%. The second study found that in a well-defined subpopulation of 1,145 adopted sons, those sons whose biological fathers had been criminals had a relatively marked tendency themselves to be criminal (in comparison to appropriate controls). These results in combination with the twin fndings suggested some genetic factor in criminality. In this project we have extended the twin population to include all twins born in Denmark between 1881 and 1920 (N equals 13,500). We have also extended the adoptee population to include all of the non-familial adoptions between 1924-1947, in all of Denmark (N equals 14,427). Analyses of the total population support the conclusions of the pilot studies. We have performed preliminary analyses on specificity of the genetic effect to certain types of crime. Certain kinds of criminal behavior seem to be genetically influenced to a greater extent than others. We intend to continue the examination by type and severity of criminal behavior as well as to examine the effect of the social and other environmental influences which might interact with the genetic factors.